In vitro transformation of murine B-lymphocytes by Abelson virus was studied and a series of splenic B-lymphocytic cell lines with antigen-binding properties were obtained. Fusion of these cell lines with plasmacytoma cells enhanced the immunoglobulin production of B-cells indicating the production of immunoglobulin gene deregulating factor by plasma cells. Methods have been developed for the fractionation of mouse embryonic DNA for the identification and cloning of endogenous type C viral genes in E. coli. Methods have also been standardized for the sequence analysis of DNA molecules.